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                <h1 id="运行时数据区"><a href="#运行时数据区" class="headerlink" title="运行时数据区"></a>运行时数据区</h1><blockquote>
<p><strong>红色区域是一个进程一份</strong>，<strong>共享的</strong>：</p>
<ul>
<li>堆</li>
<li>堆外内存(方法区)（永久代，元空间，代码缓存）</li>
</ul>
<p><strong>灰色区域是一个线程一份</strong>，<strong>私有的</strong></p>
<ul>
<li>程序计数器</li>
<li>栈</li>
<li>本地栈</li>
</ul>
</blockquote>
<p><img src="https://s1.ax1x.com/2020/10/09/0rBTED.png" alt="运行时数据区"></p>
<h2 id="线程"><a href="#线程" class="headerlink" title="线程"></a>线程</h2><blockquote>
<p>线程是一个程序里的运行单元，JVM允许一个应用有多个线程并行的执行</p>
<ul>
<li>hotspot虚拟机中，每个线程都与操作系统的本地线程直接映射，当java线程准备好以后，会创建一个操作系统的本地线程。java线程终止以后，本地线程也会回收。</li>
<li>操作系统负责把线程调度到可用的cpu上，去调用java线程中的run()方法</li>
<li>当最后一个普通线程结束的时候（main方法）虚拟机也会被关闭</li>
</ul>
</blockquote>
<h2 id="程序计数器-PC寄存器"><a href="#程序计数器-PC寄存器" class="headerlink" title="程序计数器(PC寄存器)"></a>程序计数器(PC寄存器)</h2><h3 id="介绍"><a href="#介绍" class="headerlink" title="介绍"></a>介绍</h3><blockquote>
<p>不是cpu寄存器，是jvm中对物理pc寄存器的一种抽象。</p>
<p>pc寄存器用来存储指向下一条指令的地址，也即将要执行的指令代码。由执行引擎读取下一条指令</p>
</blockquote>
<p><img src="https://s1.ax1x.com/2020/10/09/0rBOgI.png" alt="PC寄存器"></p>
<blockquote>
<p>他是线程私有的，生命周期与线程的生命周期保持一致。</p>
<p>任何时间一个线程都只有一个方法在执行，也就是所谓 的<strong>当前方法</strong>。程序计数器会存储当前线程正在执行的java方法的jvm指令地址。或者如果是在执行native方法，则是未指定值(undefined)</p>
</blockquote>
<p><strong>程序计数器是唯一一个在java虚拟机中没有规定任何OutOfMemoryError情况的区域</strong></p>
<h3 id="举例说明"><a href="#举例说明" class="headerlink" title="举例说明"></a>举例说明</h3><p><img src="/home/zhy/.config/Typora/typora-user-images/image-20200925180521546.png" alt="image-20200925180521546"></p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    <span class="token keyword">int</span> j <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
    <span class="token keyword">int</span> k <span class="token operator">=</span> i<span class="token operator">+</span>j<span class="token punctuation">;</span>
    <span class="token class-name">String</span> s <span class="token operator">=</span> <span class="token string">"abc"</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h3 id="常见问题"><a href="#常见问题" class="headerlink" title="常见问题"></a>常见问题</h3><blockquote>
<p>使用PC寄存器存储字节码指令地址有什么用呢？</p>
</blockquote>
<p>cpu需要不停的切换各个线程，这时候切换回来后，就得知道接着从哪里开始继续执行。</p>
<blockquote>
<p>为什么使用pc寄存器记录当前线程都执行地址呢？</p>
</blockquote>
<p>JVM的字节码解释器就需要通过改变PC寄存器的值来明确下一条应该执行什么样的字节码指令</p>
<h2 id="虚拟机栈"><a href="#虚拟机栈" class="headerlink" title="虚拟机栈"></a>虚拟机栈</h2><h3 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h3><blockquote>
<p>虚拟机栈跨平台容易，性能要比寄存器的低。</p>
<ul>
<li><strong>每个线程在创建的时候都会创建一个虚拟机栈</strong>，其内部保存一个个的栈帧，对应着一次次的java方法调用</li>
<li><strong>是线程私有的</strong></li>
<li>生命周期和线程一致</li>
<li>主管java程序的运行，保存方法的局部变量（8种基本数据类型，对象的引用地址），部分结果，并参与方法的调用和返回</li>
</ul>
</blockquote>
<p><strong>特点</strong>：</p>
<ul>
<li>不存在垃圾回收问题</li>
<li>每个方法的执行都伴随着压栈</li>
<li>执行结束后会有出栈工作</li>
</ul>
<p><strong>栈中出现的异常</strong></p>
<blockquote>
<p>java虚拟机规范<strong>java栈的大小是动态的或者固定不变的</strong></p>
<ul>
<li><strong>采用固定大小的虚拟机栈</strong>，在线程创建的时候，要考虑自己的局部变量表什么的是否超过了java虚拟机栈允许的最大容量，此时会抛出一个StackOverflowError</li>
<li><strong>采用动态扩展的虚拟机栈</strong>，在尝试扩展的时候没办法申请到足够的内存，或者在创建的时候就没有足够的内存去创建虚拟机栈，那么会抛出一个OutOfMemoryError</li>
</ul>
</blockquote>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">StackErrorTest</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token function">main</span><span class="token punctuation">(</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>以上代码会抛出异常：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Exception</span> in thread <span class="token string">"main"</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>lang<span class="token punctuation">.</span></span>StackOverflowError</span>
	at <span class="token class-name"><span class="token namespace">com<span class="token punctuation">.</span>study<span class="token punctuation">.</span></span>StackErrorTest</span><span class="token punctuation">.</span><span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">StackErrorTest</span><span class="token punctuation">.</span>java<span class="token operator">:</span><span class="token number">10</span><span class="token punctuation">)</span>
    <span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
<p><font color="red">我们可以通过<code>-Xss 具体内存</code>来控制栈内存的大小</font></p>
<h3 id="栈的存储单位"><a href="#栈的存储单位" class="headerlink" title="栈的存储单位"></a>栈的存储单位</h3><blockquote>
<p>栈里的数据都是以栈帧(Stack Frame)的格式存在的。</p>
<p>在线程上正在执行的每个方法都各自对应一个栈帧</p>
<p>栈帧是一个内存区块，数据集</p>
</blockquote>
<p><img src="https://s1.ax1x.com/2020/10/09/0BLxYj.png" alt="当前栈帧"></p>
<ul>
<li><p>不同的线程之间所包含的栈帧是不能相互调用的</p>
</li>
<li><p>如果当前方法调用了其他方法，方法返回的时候会把结果给前一个栈帧，自己被弹出栈</p>
</li>
<li><p>无论是正常的return返回，还是抛出异常结束（没有处理掉），都会导致栈帧被弹出</p>
<blockquote>
<p>如图中所示，当方法4出现异常，自身没有处理，栈就会弹出方法4,把异常返回给方法3，看看方法3是否处理，如果还没有处理，则栈弹出方法3并继续返回，直到最后方法1。如果方法1没有处理，那么虚拟机就会挂掉。</p>
</blockquote>
</li>
</ul>
<h3 id="栈桢的内部结构"><a href="#栈桢的内部结构" class="headerlink" title="栈桢的内部结构"></a>栈桢的内部结构</h3><blockquote>
<ul>
<li><strong>局部变量表</strong></li>
<li><strong>操作数栈</strong></li>
<li>动态链接（<strong>或指向运行时常量池的方法引用</strong>）</li>
<li>方法返回地址（<strong>或方法正常退出或者异常退出的定义</strong>）</li>
<li>一些附加信息</li>
</ul>
</blockquote>
<p><img src="https://s1.ax1x.com/2020/09/25/0ClVv6.png" alt="0ClVv6.png"></p>
<h4 id="局部变量表"><a href="#局部变量表" class="headerlink" title="局部变量表"></a>局部变量表</h4><blockquote>
<p>被称为局部变量数组或者本地变量表</p>
<p><font color="red">定义为一个数字数组，主要用于存储方法参数和自定义在方法体内的局部变量</font>,这些数据类型包括各类基本数据类型，对象引用，以及returnAddress类型</p>
<ul>
<li>建立在线程的栈上，是线程私有数据，<font color="red">不存在线程安全问题</font></li>
<li><font color="red">局部变量表所需的容量大小事在编译的时候就确定下来的</font>，并且保存在方法的Code属性的maximun local variables数据项中。运行期间不会改变局部变量表的大小</li>
<li>局部变量表的变量只在当前方法调用中有效，调用结束后，随着方法栈帧的销毁，局部变量表也会随之销毁</li>
</ul>
</blockquote>
<p><strong>slot</strong></p>
<blockquote>
<p>slot 是局部变量表中最基本的存储单元</p>
<p>在局部变量表中存放编译期可知道各种数据类型，引用类型(reference)，returnAddress类型的变量</p>
<p>32位以内的类型只占用一个slot，64位的站俩</p>
</blockquote>
<p><strong>一个问题：</strong></p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">test123</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token comment">// 这里的this会导致编译失败，因为静态方法没有this存在局部变量表中</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span>

<span class="token comment">// 相反的，在我们的实例创建的方法，或者构造方法中，this会存放在index为0的slot中</span>
<span class="token keyword">public</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token function">sout</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token comment">//是可以的</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<blockquote>
<p><strong>slot</strong>重复利用问题</p>
<p><font color = "red">栈帧中的局部变量表中的slot是可以重用的，</font>如果一个局部变量过了他的作用域，在其后声明的局部变量就可能会使用他的槽位，从而<strong>节省空间</strong></p>
</blockquote>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">/**
因为b的生命域就在&#123;&#125;中，出了&#123;&#125;他就被销毁了。但是在之前虚拟机栈已经为他开辟了空间，销毁后让c使用他的位置
*/</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">int</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#123;</span>
        <span class="token keyword">int</span> b <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        b <span class="token operator">=</span> a <span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">int</span> c <span class="token operator">=</span> a <span class="token operator">+</span> a<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>


<h4 id="操作数栈"><a href="#操作数栈" class="headerlink" title="操作数栈"></a>操作数栈</h4><blockquote>
<p>特点：后进先出</p>
<p>在方法执行的过程当中，根据<strong>字节码指令</strong>，往栈中写入数据或者提取数据，（入栈/出栈）</p>
<pre><code>- 比如执行复制，交换，求和等操作</code></pre>
<p>如果被调用的方法带有返回值，返回值会被压入当前栈帧的操作数栈中，并更新pc寄存器中下一条需要执行的字节码指令。</p>
<ul>
<li>虚拟机在操作数栈中存储数据的方式和在局部变量区中是一样的：如int、long、float、double、reference和returnType的存储。对于byte、short以及char类型的值在压入到操作数栈之前，也会被转换为int。</li>
</ul>
</blockquote>
<p><strong>虚拟机把操作数栈作为它的工作区——大多数指令都要从这里弹出数据，执行运算，然后把结果压回操作数栈。</strong></p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java">begin  
iload_0    <span class="token comment">// push the int in local variable 0 onto the stack  </span>
iload_1    <span class="token comment">// push the int in local variable 1 onto the stack  </span>
iadd       <span class="token comment">// pop two ints, add them, push result  </span>
istore_2   <span class="token comment">// pop int, store into local variable 2  </span>
end  <span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<blockquote>
<p>在上边的例子中，iload_0和iload_1把局部变量表中索引为0和1的整数压入操作数栈</p>
<p>调用iadd指令把两个数从操作数栈中弹出并相加，<strong>把结果压入操作数栈</strong></p>
<p>istore_2从操作数栈中弹出结果，把它存储到局部变量表中索引为2的位置。</p>
</blockquote>
<p><img src="http://dl.iteye.com/upload/picture/pic/116487/4032d1b9-43c5-3dc6-95c8-38b9fbb149b0.jpg" alt="演示图"></p>
<h5 id="栈顶缓存技术"><a href="#栈顶缓存技术" class="headerlink" title="栈顶缓存技术"></a>栈顶缓存技术</h5><blockquote>
<p>基于栈式架构的虚拟机使用的零地址指令更加紧凑，但是完成一个操作需要不停的入栈出栈，，经常对内存进行读写，效率会被降低。</p>
<p>所以开发者<strong>将栈顶元素全部缓存在物理cpu寄存器中，以此降低对内存的读写次数，提升执行引擎的执行效率</strong></p>
</blockquote>
<h4 id="动态链接"><a href="#动态链接" class="headerlink" title="动态链接"></a>动态链接</h4><blockquote>
<p><strong>在某些书上 动态链接，方法返回地址，附加信息会被称为</strong><font color = "red"><strong>帧数据区</strong></font></p>
</blockquote>
<ul>
<li>动态链接主要就是<strong>指向运行时常量池的方法引用</strong></li>
<li>每一个栈帧内存都包含一个指向运行时常量池中该栈帧所属方法的引用，包含这个引用的目的就是为了支持当前方法的代码能够实现动态链接(Dynamic Linking)。比如invokedynamic 指令</li>
<li>在Java源文件被编译到字节码文件中时，所有的变量和方法引用都<strong>作为符号引用保存在class文件的常量池里</strong>。比如，描述一个方法调用其他方法时，就是通过常量池中指向方法的符号引用来表示的，那么<strong>动态链接的作用就是为了将这些符号引用转换为调用方法的直接引用</strong></li>
</ul>
<p><img src="https://s1.ax1x.com/2020/10/09/0rDVK0.png" alt="0rDVK0.png"></p>
<h4 id="方法的调用：解析与分派"><a href="#方法的调用：解析与分派" class="headerlink" title="方法的调用：解析与分派"></a>方法的调用：解析与分派</h4><blockquote>
<p><strong>将符号引用转换成调用方法的直接引用</strong></p>
<p>如果<strong>转换的过程是在编译期间确定下来</strong>，并且运行期间保持不变，那么这就是<strong>静态链接</strong></p>
<p>如果<strong>被调用的方法在编译期无法被确定下来</strong>，那么只能在程序运行期间进行转换，被称为<strong>动态链接</strong></p>
</blockquote>
<p><strong>对于方法来说</strong>：</p>
<p>接口的方法调用在编译期间是不确定的，所以是<strong>晚期绑定</strong></p>
<p>而方法中如果调用了构造器，是可以在编译期间确定的，是<strong>早期绑定</strong></p>
<p><strong>虚方法和非虚方法</strong>：</p>
<ul>
<li>静态方法，私有方法，final方法，构造器，父类方法都是非虚方法</li>
<li>方法在编译器就确定了具体的调用，并且在运行时不可变，就是非虚方法</li>
</ul>
<blockquote>
<p>JVM提供了如下方法调用指令：</p>
</blockquote>
<ol>
<li>invokestatic 调用静态方法</li>
<li>invokespecial 调用私有实例方法、构造器，以及使用super关键字调用父类的实例方法或构造器，和所有实现接口的默认方法。</li>
<li>invokevirtual 调用虚方法即非私有的实例方法。</li>
<li>invokeinterface 调用接口方法，在运行时再确定一个实现此接口的对象。</li>
<li>invokedynamic 调用动态方法，在运行时动态解析出调用点限定符所引用的方法之后，调用该方法。</li>
</ol>
<p><strong>方法重写的本质</strong></p>
<p><img src="/home/zhy/.config/Typora/typora-user-images/image-20200927113251358.png" alt="image-20200927113251358"></p>
<p><strong>虚方法表</strong></p>
<blockquote>
<p>在面向对象的编程中，会很频繁的使用到动态分派，如果在每次动态分派的过程中都要重新在类的方法元数据中搜索合适的目标的话就会影响到执行效率。</p>
<p><strong>为了提高性能</strong>，jvm在类的方法区建立一个虚方法表。使用索引来代替查找。</p>
<p>每个类都有一个虚方法表，用来存放各个方法的实际入口</p>
</blockquote>
<p><strong>虚方法表在什么时候会被创建</strong>？？</p>
<blockquote>
<p>虚方法表会在类加载到链接阶段被创建并开始初始化，类的变量初始值准备完成后，JVM会把该类的方法表也初始化完毕。</p>
</blockquote>
<h4 id="方法返回地址"><a href="#方法返回地址" class="headerlink" title="方法返回地址"></a>方法返回地址</h4><blockquote>
<p>这一块区域是用来存储pc寄存器中的值，目的是为了给执行引擎让他继续做操作</p>
<p>一个方法如果是正常执行完成时<strong>pc寄存器的值作为返回地址，即调用该方法的指令的下一条指令的地址</strong>。如果是通过异常退出的，<strong>返回地址要通过异常表来确定，栈帧中一般不会保存此信息</strong></p>
</blockquote>
<p>流程：</p>
<p><img src="C:\Users\Administrator\AppData\Roaming\Typora\typora-user-images\image-20201008204215341.png" alt="image-20201008204215341"></p>
<p><strong>字节码指令：</strong></p>
<p><img src="/home/zhy/.config/Typora/typora-user-images/image-20200927115358502.png" alt="image-20200927115358502"></p>
<h4 id="一些附加信息"><a href="#一些附加信息" class="headerlink" title="一些附加信息"></a>一些附加信息</h4><blockquote>
<p>无</p>
</blockquote>
<h4 id="相关面试问题"><a href="#相关面试问题" class="headerlink" title="相关面试问题"></a>相关面试问题</h4><blockquote>
<p>为什么要使用常量池？</p>
</blockquote>
<ul>
<li>常量池的作用就是为了提供一些符号和常量，便于指令的识别。</li>
</ul>
<blockquote>
<p>举例栈溢出的情况(StackOverflowError)</p>
</blockquote>
<ul>
<li>通过-Xss设置栈的大小</li>
<li>OOM</li>
</ul>
<blockquote>
<p>分配的栈内存越大越好？</p>
</blockquote>
<ul>
<li>内存空间有限</li>
<li>不能避免栈溢出的情况发生</li>
</ul>
<blockquote>
<p>垃圾回收涉及到什么</p>
</blockquote>
<ul>
<li>方法区</li>
<li>堆</li>
</ul>
<h2 id="本地方法栈"><a href="#本地方法栈" class="headerlink" title="本地方法栈"></a>本地方法栈</h2><blockquote>
<p>native method就是java调用非java代码的接口</p>
<p>本地接口的初衷就是融合不同的编程语言为java所用</p>
</blockquote>
<p><img src="C:\Users\Administrator\AppData\Roaming\Typora\typora-user-images\image-20201008210718117.png" alt="image-20201008210718117"></p>
<blockquote>
<p>本地方法栈是用于管理本地方法的调用</p>
<ul>
<li>本地方法是线程私有的</li>
<li>允许被实现成固定或者是可动态扩展的内存大小<ul>
<li>如果线程请求分配的栈容量超过本地方法栈允许的最大容量报错栈溢出</li>
<li>如果本地方法栈可以动态扩展，在尝试扩展的时候无法申请到足够的内存就会内存溢出</li>
</ul>
</li>
<li>本地方法是c实现的</li>
<li>在本地方法栈中登记native方法，在执行引擎执行时候加载本地方法库</li>
</ul>
</blockquote>
<p><img src="C:\Users\Administrator\AppData\Roaming\Typora\typora-user-images\image-20201008213600601.png" alt="image-20201008213600601"></p>
<blockquote>
<ul>
<li>当某个线程调用一个本地方法时，他会进入一个全新的并且不受虚拟机限制的世界<ul>
<li>本地方法可以通过本地方法接口来访问虚拟机内部的运行时数据区</li>
<li>甚至可以直接使用本地处理器中的寄存器</li>
<li>直接从本地的堆中分配任意数量的内存</li>
</ul>
</li>
<li>并不是所有的JVM都支持本地方法，因为Java虚拟机规范没有明确要求本地方法站的使用语言，具体实现方式，数据结构。</li>
<li>在 Hotspot虚拟机中，直接将本地方法栈和虚拟机合二为一</li>
</ul>
</blockquote>

                
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